Saturday, 20 October 2012

The First Outlaws? Megafauna Murderers of the Wild West: Clovis Hunters

The late Pleistocene extinction of megafauna in North America was a unique event as previously, species that became extinct were small mammals. In the Pleistocene interglacial, extinction was almost entirely confined to large mammals (>1,000 kg) and medium-large ones (100 – 1,000kg) (Stuart 1991). North America lost 35 out of 45 megafauna genera during the Pleistocene (Grayson 2007: See Table).

Grayson (2007): Table of Extinct Megafauna of the Late Pleistocene

Proponents of the ‘overkill’ or ‘blitzkrieg’ hypothesis emphasize that the megafauna extinctions of 11.5 - 10,000 radiocarbon years ago coincided with the arrival of humans with advanced hunting techniques in North America, called the Clovis hunters. This could explain why megafauna extinctions were concentrated in the late Pleistocene rather than previous inter-glacial transition periods, as the only differentiating element of the late Pleistocene was the presence of the first ‘anatomically modern’ humans. Since it was the animals’ first encounter with them, they did not yet have the behavioural skills to avoid predation. Paul Martin (1967) was one of the earliest and most enthusiastic proponents of this view. Haynes (1967) showed that the earliest known appearance of Clovis hunters was 11.5-11,000 years ago, although more recent evidence suggests that an earlier people before Clovis existed as early as 13,800 years ago and was already developing the tools that would later evolve into those of Clovis hunters (See photograph). Barnosky et al (2004) points out that most large-bodied survivors of the late Pleistocene extinction event were nocturnal, alpine or deep forest dwellers, making them more difficult targets for humans. Furthermore, Graham and Lundelius (1984) suggest that the killing of megaherbivores like mammoths and mastodons (the 2 mammals with the largest number of recorded kill sites) was especially damaging and upset the entire ecosystem, driving other animals which were not directly hunted by humans to extinction. Mammoths and mastodons, like modern day elephants, tore down trees and bushes while feeding, making them accessible to other herbivores who would not otherwise be able to reach them. The disappearance of herbivores would then result in the starvation of their predators. This could be a counter-argument to Grayson and Meltzer’s (2002) argument that only 2 megafauna genera – the woolly mammoth and mastodon – have been shown to be the prey of Clovis humans, and this is supported by just 14 kill sites.

National Geographic (2011): CT scan showing cross-section of the broken tip of a spear embedded in a mastodon's rib (dated 13,800 years ago)

Grayson and Meltzer (2002) argue against this generally accepted view of human predation driving megafauna extinction. They show that only 15 of the 35 genera that were supposedly killed by Clovis hunters actually survived to 11,000 radiocarbon years ago, which is when they would have encountered Clovis hunters. Besides, megafauna extinction in North America is by no means a unique phenomenon. The entire Northern Hemisphere experienced substantial species extinctions around 10,000 – 11,000 radiocarbon years ago. In Eurasia for example, many megafauna like reindeer, giant deer and mammoth also disappeared around the same time as those in North America did. Woodman et al (1997) support the view that humans cannot be blamed for giant deer extinction in Ireland as there were no people there during that time.

The reasons for why climate change has been rejected as a cause of megafauna extinction are encapsulated in Fiedel’s (2009:30) question: “if Holocene warming was so disastrous for megafauna, why wasn't there a wave of extinction around 125 ka in the last interglacial?” There are 2 arguments proponents of the climate change explanation use. The first is that the last interglacial was much warmer than previous interglacials and had fewer seasonal temperature variations, leading to mosaic vegetation types that supported a richer range of animal species (Graham and Lundelius 1984). When the open grasslands of the late Pleistocene gave way to thicker forests and vegetation which showed strong zoning according to climate during the Holocene, animal extinctions occurred as the less diverse vegetation could no longer support the previous level of biodiversity (Stuart 1991). Graham (1985) has also argued that the Pleistocene – Holocene transition was unique from previous transitions. Not only was Holocene climate and vegetation was very different from that of previous interglacial periods, but also unusually, the last interglacial was warmer, rather than cooler, than the Holocene in both North America and Europe.

The second is that the late Pleistocene was biotically unique. Scott (2010) argues that this assumes a static view of megafauna composition and hence, similar adaptations to climate change. He uses the example of bison becoming more widespread in North America. Guthrie (1980) proposed that since bison survived while other species perished during this period, this may suggest that competition from bison, along with failure to adapt to climate change, drove the megafaunal extinction event. Nevertheless, the key takeway from Scott’s paper is that ‘megafauna’ is not a static concept, and this has implications for evolutionary responses to climate change. This squares with Graham and Lundelius’ (1984) theory of ‘coevolutionary disequilibrium’ – species evolve individualistically and uniquely to climate change in ways that may become detrimental to other species.

Additionally, during 10,000 years of the Holocene very few land mammals became extinct, although human populations certainly increased dramatically (Stuart 1991).

Nevertheless, the extinctions in North America were definitely more sudden and severe than in Europe. The North American case supports the view of a greater, but not dominant, human role in extinction in relation to other regions. The arrival of Clovis hunters (not present in Europe) at a time of more profound climate change was probably the last straw for many species already suffering the stress from a changing environment.